Aluminium is an often used material for manufacture of products by brazing. Aluminium can be alloyed by adding various alloying elements, such as Mn, Mg, Ti, Si, and the strength of the aluminium alloy is affected by precipitation of particles or by the alloying materials forming a solid solution with aluminium.
Material for brazing of the above type can be given high strength after the brazing by cold working it prior to the brazing, that is, rolling or stretching at temperature below 200° C., which increases the strength, and doing this in a way so that it does not lose the strength increase upon brazing. This means that the material is prevented from recrystallising entirely during the heat treatment which brazing entails. Such a material, furthermore, can be given high resistance to fatigue and creep when used at high temperature, up to and including 300° C. This high strength at high temperature is created both by lowering the driving force for recrystallisation by selecting a sufficiently low degree of deformation during the cold working and by increasing the retarding force by creating a sufficiently large quantity of particles per unit of volume.
Material for brazing can be coated with a braze layer of an alloy with high silicon content. In brazing, such material is placed in contact with another part and heated in a brazing furnace. The high silicon content in the braze layer causes the braze layer to melt at lower temperature than the underlying core layer, flow away due to capillary forces and surface tension differences, and form brazed seams with the other part.
Another variant of brazing material does not have any braze layer, but it is brazed to a material with such a layer. For example, such material can be used in so-called fins in heat exchangers, such as automobile radiators that are folded from thin aluminium sheet. When fabricating the heat exchanger, the fins are placed against braze-clad tubes and then heated in a brazing furnace so that the braze layer on the tube melts and flows away due to capillary forces and surface tension differences and forms brazed joints between the fins and the tube.
The primary function of the fins in a gas/liquid heat exchanger, such as an automobile radiator, is to conduct the heat from the liquid in the tube to the gas. The fins often have additional tasks. The brazing is done at very high temperature, so that the material can be creep-deformed only by the mechanical stress caused by its own weight. The fins must not become so soft that they collapse, but rather help to maintain the form of the heat exchanger. The capacity of the fins in this regard, their “sagging resistance”, is measured by fastening strips of a certain length, such as 50 mm, horizontally at one end in a furnace that is heated up to 600° C. The sagging of the free end is measured when the furnace has cooled down. It is also important to the ability of the heat exchanger to resist the high pressure which can occur in the tube during operation that the fins help out in resisting this high pressure. If the fins have good strength also at high temperature, the tube can be thinner, which means that the heat exchanger weighs less.
If the material of parts being brazed is not recrystallized when heated to the brazing temperature before the melting temperature of the braze is reached, silicon from the braze will penetrate the material being brazed. This entails a risk, in thin sheet such as fins, of melting and collapsing or, alternatively, of forming incomplete or poorly brazed seams with large pores. The penetration of the silicon occurs by diffusion, melting of the outer layer, or so-called “liquid film migration” [see, e.g., A. Wittebrod, S. Desikan, R. Boom, L. Katgerman, Materials Science Forum Vols. 519-521, (2006) pp. 1151-1156)].
Therefore, a brazing material per the above that does not recrystallize during brazing must have a barrier layer. A suitable name for a material consisting of several layers is sandwich material. The function of the barrier layer is to reduce the penetration of silicon from the braze material into the underlying core material during the brazing and thereby assure the formation of good brazed joints, so that the core material does not begin to melt. Silicon penetration occurs especially easy at grain boundaries. Therefore, large grains need to be formed in the barrier layer so that there are few grain boundaries.
One problem with ordinary high-strength brazing material, such as that with high contents of manganese, is that its corrosion properties are not the best. Intermetallic particles with iron, manganese, and aluminium are more noble than the surrounding aluminium matrix, which gives rise to pitting in moist settings. Commercially pure aluminium having only iron and silicon as alloying material and also low iron content has much better properties in this regard. Barrier layer and core layer can therefore advisedly be constituted such that the sandwich material has good corrosion properties.
If the tubes in air/water heat exchangers become corroded, they will leak, which must be prevented. Therefore, zinc is often added to the alloy in the fins so that they have lower electrical potential in relation to the tube and provide so-called cathodic protection. Of course, this results in greater overall corrosion on the fins. But this may be acceptable, whereas intergranular corrosion and pitting which leads to a faster dissolving of the fins must not occur. One way of further improving the corrosion properties is to increase the electrochemical potential of the core layer. This can be done, for example, by using copper, manganese or some other alloying material that increases the electrochemical potential in solid solution and that is placed in solid solution by the brazing process.
One problem with known types of brazing material is that they lack sufficient fatigue strength and creep resistance at high temperatures. If the temperature is high during the testing, over 200° C., and the material is subjected to high stress also the lifetime for fatigue stressing of the material's creep resistance will be limited. Since the intermetallic precipitations contribute greatly to the strength at high temperatures, it is important for them to be stable and not dissolve too fast over time. This is especially important for a core material that is not recrystallized, since the precipitates retard the course of recrystallisation.
Two examples of products that need better fatigue strength and creep resistance at temperatures over 150° C. and up to 300° C. are intercoolers and exhaust gas coolers for recycling in automobile engines. These products are usually fabricated by brazing of sandwich material. Increased demands on automobile engines for reduced emissions of polluting gases and improved efficiency means that these coolers are subjected to increasingly higher operating temperatures and gas pressures. This causes a problem, since existing sandwich material does not meet the strength requirements. Ordinary automobile radiators that do not reach operating temperature higher than 100° C. are today made in relatively heavy material dimensions for strength reasons. The heavy weight contributes to high fuel consumption. The large quantity of material used in radiators also makes them costly to manufacture. Even though the fins are thin as compared to tubes and other parts in an automobile radiator, they still amount to a large part of the radiator's weight, perhaps 40%, and it is therefore very important for them to have good strength at the operating temperature so that their thickness can be reduced.
The above problem has been solved for tubes and end plates of heat exchangers by the method specified in WO 2009/128766. In this method, the core layer has a composition such that it does not recrystallize during brazing. To prevent silicon from the braze penetrating into the core layer, a barrier layer is applied by rolling, consisting of an aluminium alloy that recrystallizes in large grains during brazing. One problem is that it can be hard to get the barrier layer to adhere to the core layer during hot rolling if there is a large difference in deformation resistance between core layer and barrier layer and if the barrier layer is very thick. An oxide-free aluminium surface is very quickly covered by oxide when it makes contact with air. To get adhesion, a metal surface without oxide needs to be created both on core layer and barrier layer so that one gets a metal against metal contact. This is achieved by the surface enlargement produced by rolling if both layers are deformed. For example, if the core layer is much harder than the barrier layer, then the core layer will not be deformed.
In the fabrication process, plates of the barrier layer are placed on one or both sides of an ingot of the core alloy. For good yield in the industrial rolling process, the combined thickness of this sandwich pack is 60 cm thick. It is then necessary to start the rolling with relatively small reductions in each rolling pass. Since the ratio between the diameter of the working rolls and the thickness of the sandwich pack is small, this means that the primary thickness reduction and thus the surface enlargement occurs near the surfaces of the sandwich pack. If the barrier layer is thick, the surface enlargement is small in the boundary layer between barrier layer and core ingot and it is hard to make the layers stick together. An even greater problem is that most of the thickness reduction occurs at the surface, so the barrier layer is lengthened more than the core layer. This squeezes the barrier layer out both to the front and rear of the core layer. These projecting parts must then be trimmed off, which lowers the efficiency of the process. Furthermore, the barrier layer is forced out to the sides beyond the core layer, which means one gets a variation in thickness of the barrier layer over the width of the finished sheet. The edges of the rolled sheet must therefore be sheared off and scrapped, since their thickness is too thin at the barrier layer. This further decreases the yield of the process. Of course, if the barrier layer is softer than the core layer, which is often the case, the problem of poor yield is further accentuated. This problem becomes even more severe in very thin sheet, such as heat exchanger fins, which are often thinner than 0.1 mm and can be as thin as 0.05 mm. This means that for the barrier layer to work, which requires a thickness of at least 0.007 mm, it will take up a sizeable part of the thickness. It is then hard for the customary method of fabricating thin sheets for heat exchangers—hot rolling—to have a good yield, especially if the core layer is much harder than the barrier layer. If the barrier layer is thicker than 20% of the total thickness, it is hard to make the layers stick together at all during the rolling.
What primarily makes a material hard during rolling is its content of many hard intermetallic particles. Alloy elements in solid solution also increase the resistance to deformation. In a sandwich material, the core layer should have many particles so as not to recrystallize, while the barrier layer should have few particles, so as to recrystallize in a large grain size at a relatively low temperature. Thus, the hardness difference between the layers can be large when they are rolled together, and this must be avoided in order to get a good yield.